Optical amplifier, method of controlling the output light from the optical amplifier, optical transmission system and method of controlling an optical transmission path

ABSTRACT

In an optical amplifier 8 or 10, multiplexed light beams inputted to an optical amplifying medium 19 are guided and the optical amplifying medium 19 is excited by an exciting source 21 to change the mu-factor. Part of multiplexed light beams amplified by the optical amplifying medium 19 are branched by an optical coupler, only a probe light beam is separated from the branched multiplexed light beams by means of a light separating unit 24, and the probe light beam is detected by a light receiving unit 28. A controller 29 controls an amount of operation of the exciting source 21 on the optical amplifying medium 19 such that an output of the light receiving unit 28 (an output of the probe light beam) becomes constant, thereby controlling the output of the probe light beam and the outputs of the multiplexed signal light beams to constant values. By using the optical amplifier 8 or 10 constructed as above, an optical amplifier can be provided which can control individual wavelength outputs without greatly affecting the signal transmission even when the number of signal wavelengths to be multiplexed changes and an optical transmission system using the optical amplifier can also be provided.

BACKGROUND OF THE INVENTION

The present invention relates to an optical amplifier used in amultiplexed optical transmission path, a method of controlling amultiplexed light output delivered out of the optical amplifier, anoptical transmission system using the optical amplifier and a method ofmonitoring and controlling the optical transmission path fortransmission of the multiplexed light output.

In recent years, with the demand for reducing the cost of an opticalcommunication system, a so-called wavelength multiplexing opticaltransmission scheme has been studied in which two or more kinds ofsignal light beams of different wavelengths are multiplexed andtransmitted through a single optical transmission fiber. Since theoptical amplifier has a wide amplifying wavelength band and can affordto perform amplification at low noise, it is suitable for use as anamplifier in the wavelength multiplexing optical transmission. A rareearth added optical fiber constituting the optical amplifier and asemiconductor optical amplifier have each the gain which has wavelengthdependency and therefore, there occurs a difference in light output orgain between wavelengths after amplification. Specifically, theinter-wavelength difference is accumulated in the course of multi-stagerelay based on optical amplifiers and after the relay, the difference inoptical power between wavelengths is extended. As a result, the maximumrelay transmission distance of the whole system is limited by a degradedS/N ratio of a wavelength of the multiplexed wavelengths which has thelowest power. Accordingly, it is of importance to provide an opticalamplifier which does not cause the light output difference betweenwavelengths.

Thus, for example, a scheme described as "Flattening of Multi-wavelengthAmplifying Characteristics in Optical Fiber Amplifier Using FiberMu-factor Control" in The Institute of Electronics and Information andCommunication Engineers of Japan, Technical Report OCS94-66, OPE94-88(1994-11) has been known as a conventional scheme.

The conventional scheme uses a fiber gain controller (AFGC) formonitoring the total output of four signal light beams subject towavelength multiplexing and controlling the fiber gain such that theoutput level becomes constant. Through this, the inter-input wavelengthdifference is made to be 0 dB and the fiber gain is controlled to aconstant value of 12 dB so as to minimize the difference betweenwavelengths. Further, by using an auto-power controller (APC) based onan optical attenuator 58, light loss is adjusted while keeping the fibergain constant at 12 dB to make the fiber gain spectrum unchanged evenwhen the relay mu-factor is changed.

Typically, in a practical system of wavelength multiplexingtransmission, transmitting signal information pieces represented bywavelengths are often independent of each other. In this case, onlynecessary signals are transmitted and there is a possibility thatunnecessary signals are stopped, that is, placed in standby condition.

However, in the conventional scheme in which the total output ofmultiplexed four signal light beams is monitored and controlled, whenthe number of multiplexed signal wavelengths is changed, the totaloutput remains unchanged but outputs of signal light beams of individualwavelengths change greatly. With the outputs of signal light beamsgreatly changed in this manner, there arise problems that signaltransmission is adversely affected and that when the outputs exceed asignal transmission distance limit, the signal transmission becomesimpossible.

Further, in the conventional scheme in which the total light output iscontrolled, when the output of one signal light beam decreases, thisdecrease adversely affects the signal transmission.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an optical amplifierwhich can control individual wavelength outputs without greatlyaffecting signal transmission even when the number of signal wavelengthssubject to multiplexing changes and an optical transmission system usingthe optical amplifier.

Another object of the present invention is to provide a method ofmonitoring a probe light beam included in a multiplexed light raypropagated through a transmission path in the optical transmissionsystem and controlling an output of the probe light beam and outputs ofthe multiplexed light beams from the optical amplifier in accordancewith a result of monitoring.

Another object of the present invention is to provide an opticaltransmission path monitoring and controlling method which can decidewhether or not an optical transmission path trunk line, a probe lightbeam per se, multiplexed light beams per se, an optical amplifier and atransmitter are normal by monitoring the probe light beam and othermultiplexed light beams included in the multiplexed light ray propagatedthrough a transmission path in an optical transmission system anddetecting what values the probe light beam and the other multiplexedlight beams take in relation to predetermined values, respectively.

To accomplish the above objects, the present invention comprises anoptical amplifying medium to which input multiplexed light beams areguided, an exciting source for exciting the optical amplifying medium,light branching means for branching part of the multiplexed light beamsamplified by the optical amplifying medium, light separating means forseparating a probe light beam from multiplexed light beams branched bythe light branching means, light receiving means for detecting an outputof the probe light beam separated by the light separating means andconverting the detected output into an electric signal, and controlmeans for controlling an amount of operation of the exciting source onthe optical medium such that an output of the light receiving meansbecomes constant, and with the construction as above, the output of theprobe light beam can be rendered to be constant, the outputs of thesignal light beams can be rendered to be constant and even when thenumber of signal wavelengths subject to multiplexing changes at thattime, the individual wavelengths can be controlled without greatlyaffecting the signal transmission.

Preferably, the above optical amplifier further comprises second lightreceiving means for detecting outputs of multiplexed light beamsbranched by the light branching means, second light branching meansarranged to precede the optical amplifying medium and operative tobranch part of the multiplexed light beams guided to the opticalamplifying medium, second light separating means for separating theprobe light beam from the multiplexed light beams branched by the secondbranching means, third and fourth light receiving means for detectingthe output of the probe light beam branched by the second lightseparating means and the outputs of the multiplexed light beams branchedby the second branching means, respectively, and input detecting meansfor delivering a command to the control means when the output of theprobe light beam detected by the third light receiving means is lowerthan a predetermined value and the outputs of the multiplexed lightbeams detected by the fourth light receiving means are higher than apredetermined value, and the control means controls the amount ofoperation of the exciting source on the optical medium such that theoutput of the second light receiving unit becomes constant, so that withthe above construction, event when the probe light beam becomesabnormal, all of the multiplexed light beams can be controlled.

Preferably, the optical amplifier further comprises alarm means forinforming abnormality, the input detecting means delivers a signal tothe alarm means when the output of the probe light beam and the outputsof the multiplexed light beams are lower than the predetermined values,respectively, so as to actuate the alarm means, and with the aboveconstruction, even when abnormality occurs, the abnormality can beinformed easily.

Preferably, in the optical amplifier, the optical amplifying medium is arare earth added optical fiber, the exciting source is a pump laser, andwith the above construction, the amplification degree for signal lightbeams can be increased.

Preferably, the optical amplifier further comprises correcting means forcalculating a difference between outputs of the first and second lightreceiving units and correcting a reference value, included in thecontrol means and used to control the probe light beam to a constantvalue, in such a manner that the difference becomes a predeterminedvalue, and with the above construction, the output during laying can bemade to be equal to that during initial adjustment.

Preferably, in the optical amplifier, the light separating meansincludes first and second optical couplers each having four ports and anoptical filter for selectively passing a light beam of a predeterminedwavelength, the multiplexed light beams branched by the light branchingmeans are guided to the first port of the first optical coupler,multiplexed light beams delivered out of the fourth port are detected bythe second light receiving means, the multiplexed light beams branchedby the light branching means are guided to the first port of the firstoptical coupler, multiplexed light beams delivered out of the third portare guided to the optical filter and are further guided to the thirdport of the second optical coupler, the probe light beam delivered outof the second port is detected by the second light receiving unit,multiplexed light beams branched by the second light branching means areguided to the first port of the second optical coupler, multiplexedlight beams delivered out of the fourth port are detected by the fourthlight receiving unit, multiplexed light beams branched by the secondlight branching means are guided to the first port of the second opticalcoupler, multiplexed light beams delivered out of the third port areguided to the optical filter and further guided to the third port of thefirst optical coupler, and the probe light beam delivered out of thesecond port is detected by the first light receiving unit, and with theabove construction, the construction of the light separator can besimplified.

Preferably, the optical amplifier further comprises combining means forcombining the probe light beam delivered out of the separating meanswith light beams in the stage preceding the optical amplifying medium,and with the above construction, even when the probe light beam becomesabnormal, the succeeding trunk line system cannot be affected thereby.

The present invention comprises a light transmitting apparatus formultiplexing a plurality of signal light beams inclusive of a probelight beam and delivering a multiplex light ray to a trunk line systemconstructed of an optical fiber, an optical amplifier provided midway ofthe trunk line system and operative to separate the probe light beamfrom the light signal delivered out of the light transmitting apparatusand control the mu-factor for the input light beams such that an outputof the probe light beam becomes constant, and a light receivingapparatus for receiving a light signal amplified by the opticalamplifier and transmitted to the trunk line system, and with the aboveconstruction, even when the number of signal wavelengths to bemultiplexed changes, signal transmission cannot be affected thereby andthe individual wavelength outputs can be controlled.

Preferably, in an optical transmission system using the opticalamplifier, the light transmitting apparatus includes a plurality oftransmitters for transmitting signal light beams and a probe light beamtransmitter for constantly transmitting the probe light beamindependently of the plurality of transmitters, and with the aboveconstruction, outputs of the signal light beams can be controlledwithout affecting the signal light beams.

Preferably, in the optical transmission system using the opticalamplifier, the light transmitting apparatus includes a plurality oftransmitters for transmitting signal light beams, one of the signallight beams transmitted from the plurality of transmitters is used asthe probe light beam, and with the above construction, the transmissionamount of information of the signal light beams can be increased withoutrequiring the provision of a separate probe light beam.

Preferably, in the optical transmission system using the opticalamplifier, the outputs of individual wavelengths from the transmittersare made to be substantially equal to each other, and with theconstruction as above, the signal light beams can be controlled withease.

Preferably, in the optical transmission system using the opticalamplifier, the output of the probe light beam is made to be higher thanthe outputs of the signal light beams from the transmitters, and withthe construction as above, even when the transmission distance of thetrunk line system is increased and input signal light beams aredecreased, the probe light beam can be detected to ensure that outputsof the signal light beams can be amplified and transmitted through thelong distance.

Preferably, in the optical transmission system using the opticalamplifier, the output of the probe light beam transmitter is made to belower than the outputs of the signal light beam transmitters, and withthe above construction, the outputs of the signal light beams can beincreased.

Preferably, in the optical transmission system using the opticalamplifier, a wavelength distance between the probe light beam and anadjacent signal light beam is made to be different from that between thesignal light beams, and with the above construction, accuracies of thelight outputs can be improved.

Preferably, in the optical transmission system using the opticalamplifier, the probe light beam has a wavelength positioned at a shortwavelength end or a long wavelength end of the signal light beams, andwith the above construction, separation of the probe light beam can befacilitated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the schematic construction of anoptical transmission system according to an embodiment of the presentinvention.

FIG. 2 is a block circuit diagram showing the detailed construction ofan optical amplifier in the first embodiment.

FIG. 3 is a block circuit diagram showing another embodiment of acontrol system for controlling the optical amplifying medium in FIG. 2.

FIG. 4 is a block circuit diagram showing another embodiment ofwavelength separating means in FIG. 2.

FIG. 5 is a block circuit diagram showing still another embodiment ofthe wavelength separating means in FIG. 2.

FIG. 6 is a block diagram showing the schematic construction of anoptical transmission system according to another embodiment of thepresent invention.

FIG. 7 is a table showing a transmission system control method of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Various embodiments of the present invention will be described hereunderin greater detail with reference to the drawings.

FIG. 1 is a system construction diagram of an optical transmissionsystem according to a practical form of the present invention.

A light transmitting apparatus 1 comprises a transmitter 2 fortransmitting a probe light beam having a wavelength λp=1545 nm, atransmitter 3 for transmitting a signal light beam of a wavelengthλ1=1547 nm, a transmitter 4 for transmitting a signal light beam ofλ2=1550 nm, a transmitter 5 for transmitting a signal light beam ofλ3=1553 nm, a transmitter 6 for transmitting a signal light beam ofλ4=1556 nm, and an optical multiplexer 7 for multiplexing the lightbeams from the transmitters 3, 4, 5 and 6 and delivering multiplexedlight beams to the same optical fiber. Here, the probe light beam λp isconstantly delivered regardless of the presence or absence of the signallight beams λ1, λ2, λ3 and λ4. While a wavelength distance betweenadjacent ones of the signal light beams λ1, λ2, λ3 and λ4 is 3 nm, awavelength distance between the probe light beam λp and the signal lightbeam λ1 in the close proximity thereof is 2 nm. A light output of theprobe light beam λp and a light output of each of the signal light beamsλ1, λ2, λ3 and λ4 are each +10 dBm.

The light beams multiplexed by the light transmitting apparatus 1 areamplified by an optical amplifier 8 and are then delivered to a trunkline system 9. At that time, the mu-factor of the optical amplifier 8 iscontrolled such that the probe light beam λp and each of the signallight beams λ1, λ2, λ3 and λ4 have each a light output power level of+10 dBm. The optical amplifier 8 branches the probe light beam from theinputted probe light beam λp and signal light beams λ1, λ2, λ3 and λ4and controls the mu-factor by performing feedback such that the outputof the probe light beam becomes constant. Since the probe light beam λpand signal light beams λ1, λ2, λ3 and λ4 which are 100 km transmittedthrough the trunk line system 9 undergo loss and variation to change toabout -20 dBm to 0 dBm, they are compensated for optical loss by anotheroptical amplifier 10 provided midway of the trunk line system 9 to bestabilized by the amplifier and thereafter, again delivered to the trunkline system 9. Light output power levels stabilized by the opticalamplifier 10 are each about +10 dBm.

Further, the multiplex light ray delivered to the trunk line system 9 is100 km transmitted and then inputted to a light receiving apparatus 11.The light receiving apparatus 11 includes an optical demultiplexer 12for demultiplexing the probe light beam λp and signal light beams λ1,λ2, λ3 and λ4 and light receivers 13, 14, 15, 16 and 17 provided for therespective wavelengths.

Here, the present optical transmission system features that the lighttransmitting apparatus 1 includes not only the transmitters of signallight beams but also the transmitter of probe light beam. As will bedescribed later with reference to FIG. 2 and ensuing figures, theoptical amplifier used in the trunk line system separates the probelight beam from the light beams transmitted in a wavelength multiplexedform through the trunk line system and controls the gain of an opticalamplifying medium included in the optical amplifier such that theintensity of the probe light beam is rendered to be constant.Accordingly, even when the number of signal light beams subject towavelength multiplexing decreases from four to three or the intensity ofone signal light beam decreases, respective wavelength outputs can becontrolled without greatly affecting the signal transmission.

Namely, according to the present practical form, even when the number ofsignals undergoing wavelength multiplexing changes or the output of onesignal light beam decreases, the respective wavelength outputs can becontrolled without greatly affecting the signal transmission.

Referring now to FIG. 2, the internal construction of the opticalamplifier will be described.

FIG. 2 is a functional block diagram of an optical amplifier accordingto a practical form of the present invention.

The probe light beam λp and signal light beams λ1, λ2, λ3 and λ4 fromthe trunk line system 9 are inputted to an input terminal of the opticalamplifier 8 or 10 through the same optical fiber. The trunk line system9 in a practical system has irregularities in light loss and distanceand therefore, the input light power changes to about -20 dBm to 0 dBm.Most of the input probe light beam λp and signal light beams λ1, λ2, λ3and λ4 pass through a 5:95 optical coupler 18 standing for first lightbranching means and fed to an optical amplifying medium 19.

The optical amplifying medium 19 includes an optical combining unit 20for combining a pump light beam from a pump laser 21 serving as anexciting source, and an erbium added optical fiber 22 for receiving alight beam combined by the optical combining unit 20. The light beamguided to an input terminal of the erbium added optical fiber 22 isamplified inside the erbium added optical fiber 22 and delivered from anoutput terminal. Most of a delivered light beam passes through a 5:95optical coupler 23 standing for second light branching means and isdelivered from an output terminal of the optical amplifier 8 or 10 tothe trunk line system 9.

Part of the light beam is branched by the 5:95 optical coupler 23 andinputted to first wavelength separating means 24. The wavelengthseparating means 24 includes a 50:50 optical coupler 25 and an opticalbandpass filter 26 having a pass wavelength band of λp±nm.

A light beam inputted from a port 1 of the 50:50 optical coupler 25 is50:50 branched from ports 4 and 3 and the probe light beam λp and thesignal light beams λ1, λ2, λ3 and λ4 are delivered. The light beamdelivered out of the port 3 of the 50:50 optical coupler 25 is inputtedfrom a port 1 of the optical band-pass filter 26 and only a light beamof λp standing for the probe light beam is passed to a port 2.

Of the thus separated light beams, the probe light beam λp and signallight beams λ1, λ2, λ3 and λ4 which are delivered out of the port 4 ofthe 50:50 optical coupler 25 are monitored by a first light receivingunit 27 and the probe light beam λp delivered out of the port 2 of theoptical band-pass filter 26 is monitored by a second light receivingunit 28.

Either one of monitor signals is selected for transfer by means of aselector 30A inside a controller 29 and is compared with a referencevoltage 31A or 31B selected by a selector 30B and corresponding to apredetermined output level by means of a comparator 32 to generate anerror signal. Structurally, a drive current of the laser pump 21 isfeedback-controlled by the error signal obtained from the comparator 32.

Here, when an input detecting unit 33 generates a first operationsignal, the selector 30A selects the monitor signal from the lightreceiving unit 28 and with a second operation signal generated, themonitor signal from the light receiving unit 27 is selected. Theselector 30B selects a reference voltage from the reference voltage 31Awhen the input detecting unit 33 generates the first operation signaland selects a reference voltage from the reference voltage 31B when thesecond operation signal is generated.

The operation of the input detecting unit 33 will be described later butit normally generates the first operation signal to control a signal ofprobe light beam detected by the light receiving unit 28 such that thesignal is rendered to be equal to the reference voltage. When anabnormality occurs in the probe light beam, the input detecting unit 33generates the second operation signal to selectively switch the selector30A or 30B.

With the above construction, when the trunk line system is normal andthe probe light beam is normally inputted, the drive current of the pumplaser 21, that is, the amount of operation on the erbium added opticalfiber serving as the optical amplifying medium of the pump laserstanding for the exciting source is feedback-controlled such that thelight output of the probe light beam λp monitored by the light receivingunit 28 can be kept to be constant. This keeps the gain state in theerbium added optical fiber 22 constant and hence makes it possible tokeep constant the optical gain states (outputs) of the plurality ofsignal light beams λ1, λ2, λ3 and λ4 which are inputted concurrently. Asa result, the light outputs of the signal light beams λ1, λ2, λ3 and λ4can each be maintained at a predetermined value and can be stabilized.

Here, the optical amplifying medium 19 may alternatively be asemiconductor optical amplifier and in this case, the pump laser 21provides an exciting current. Namely, by controlling the excitingcurrent acting as the exciting source, the amount of operation on thesemiconductor optical amplifier standing for the optical amplifyingmedium can be feedback-controlled.

On the other hand, the probe light beam λp and signal light beams λ1,λ2, λ3 and λ4 which are partly branched by the first 5:95 opticalcoupler are inputted to second wavelength separating means 34. Thewavelength separating means 34 includes a 50:50 optical coupler 35 andan optical band-pass filter 36 having a pass wavelength band of λp±1 nm.

The 50:50: optical coupler 35 passes the probe light beam λp and signallight beams λ1, λ2, λ3 and λ4 from a port 1 to a port 4. A light beamfrom a port 3 of the 50:50 optical coupler 35 is inputted to a port 1 ofan optical band-pass filter 36 so that only a light beam of λp standingfor the probe light beam may be passed through a port 2. Of the thusseparated light beams, the probe light beam λp and signal light beamsλ1, λ2, λ3 and λ4 delivered out of the port 4 of the 50:50 opticalcoupler 35 are monitored by a third light receiving unit 37 and theprobe light beam λp delivered out of the port 2 of the optical band-passfilter 36 is monitored by a fourth light receiving unit 38.

The probe light beam λp monitored by the light receiving unit 38 iscompared with a predetermined reference voltage 39 inside the inputdetecting unit 33 by means of a comparator 40 and compared informationis transmitted to an operation designating circuit 52. The probe lightbeam λp and signal light beams λ1, λ2, λ3 and λ4 which are monitored bythe light receiving unit 37 are compared with a predetermined referencevoltage 41 by means of a comparator 42 and compared information istransmitted to the operation designating circuit 52.

When the probe light beam λp is selected to a short wavelength end or along wavelength end of the signal light beams λ1, λ2, λ3 and λ4, alow-pass filter or a high-pass filter for filtering off the probe lightbeam λp from the multiplex light beam can be used as the opticalband-pass filter 26 or 36 and the construction of the optical band-passfilter can be simplified.

In this case, by using an optical separating unit as the first or secondwavelength separating means (24, 34), the construction can further besimplified.

The probe light beam λp is constantly emitted. Therefore, when the probelight beam is higher than the predetermined value and the probe lightbeam λp and signal light beams λ1 to λ4 which are monitored by the lightreceiving unit 37 are also higher than the predetermined value, theconnection state of the trunk line system 9 is determined to be good andthe operation designating circuit 52 transmits the first operationsignal to the selector 30A inside the controller 29 so that the outputsignal of the light receiving unit 28 may be so connected as to beinputted to the comparator 32. At the same time, the first operationsignal is transmitted to the selector 30B so that the reference voltage31A may be so connected as to be inputted to the comparator 32. Thus,when the probe light beam λp is higher than the predetermined value andthe probe light beam λp and signal light beams λ1 to λ4 which aremonitored by the light receiving unit 37 are higher than thepredetermined value, the output of the light receiving unit 28 iscontrolled such that it becomes equal to the reference voltage 31A andthe drive current of the pump laser 21 is feedback-controlled such thatthe optical output of the probe light beam λp can be kept to be constantas described previously.

When the probe light beam λp monitored by the light receiving unit 38 ishigher than the predetermined value and the probe light beam λp andsignal light beams λ1 to λ4 which are monitored by the light receivingunit 37 are lower than the predetermined value, it is determined thatthe signal light beam λ1, λ2, λ3 or λ4 by itself becomes abnormal andthe operation designating circuit 52 sends a third operation signal toan alarm circuit to cause the alarm circuit 53 to raise the alarm.

Alternatively, control may be performed such that the third operationsignal is used to lower the drive current of the pump laser 21 so as toprevent superfluous optical amplification.

When the probe light beam λp monitored by the light receiving unit 38 islower than the predetermined value and the probe light beam λp andsignal light beams λ1, λ2, λ3 and λ4 which are monitored by the lightreceiving unit 37 are higher than the predetermined value, it isdetermined that the probe light beam λp by itself becomes abnormal andthe operation designating circuit 52 transmits the second operationsignal to the selector 30A inside the controller 29 so that the outputsignal of the light receiving unit 27 may be so connected as to beinputted to the comparator 32 by means of the operation designating unit52. At the same time, the second operation signal is also transmitted tothe selector 30B so that the reference voltage 31B may be so connectedas to be inputted to the comparator 32. Thus, when the probe light beamλp is lower than the predetermined value, the output of the lightreceiving unit 27 is controlled such that it becomes equal to thereference voltage 31B. At that time, it is determined that the probelight beam λp by itself becomes abnormal in spite of the fact that thetrunk line system 9 is normal, and the drive current of the pump laser21 is feedback-controlled such that light outputs of the signal lightbeams λ1, λ2, λ3 and λ4 which are monitored by the light receiving unit27 can be kept to be constant. In this manner, signal light beam outputlevels can be controlled such that they are not raised abnormally in theabsence of the probe light beam λp. The state that the probe light beamis absent means a state in which the probe light beam is not deliveredfor some causes (for example, a failure of the laser). At that time,given that the transmitted signal output of each of the signal lightbeams λ1, λ2, λ3 and λ4 is 10 dBm, the signal light beams are controlledsuch that the output of each of all the four multiplexed light beams λ1,λ2, λ3 and λ4 is +16 dBm.

In the event that the probe light beam by itself is suddenly stopped,the alarm is transmitted from the operation designating circuit 52 tothe selector 30A and 30B at a speed of about 500 μs. Receiving thissignal, the selectors 30A and 30B are switched at a speed of about 50μs. Since the erbium added optical fiber 22 has a response time of about1 to 10 ms which is sufficiently slower than the alarm transmittingspeed, the optical power as a whole can be controlled without causingthe optical output to be increased abruptly and excessively.

By repairing the probe light beam while the second operation signal isgenerated, that is, while the optical outputs of the signal light beamsλ1, λ2, λ3 and λ4 are kept to be constant, a suitable system state canbe maintained without making the signal transmission abnormal andreliability of the system can be improved.

Instead of using the two reference voltages such as the referencevoltages 31A and 31B and switching them for use, a single referencevoltage can alternatively be used by changing sensitivity of the lightreceiving unit 28 and that of the light receiving unit 27 such that, forexample, an output electric signal has the same value when a lightsignal of 10 dBm is inputted to the light receiving unit 28 as when alight signal of 16 dBm is inputted to the light receiving unit 27 tothereby make the sensitivity of the light receiving unit 27 lower thanthat of the light receiving unit 28.

When the probe light beam λp monitored by the light receiving unit 38 islower than the predetermined value and the probe light beam λp andsignal light beams λ1, λ2, λ3 and λ4 which are monitored by the lightreceiving unit 37 are also lower than the predetermined value, it isdetermined that the trunk line system is abnormal and the operationdesignating circuit 52 sends the third operation signal to the alarmcircuit 53 to cause it to raise the alarm.

Alternatively, control may be performed such that the third operationsignal is used to lower the drive current of the pump laser 21 so as toprevent superfluous optical amplification.

The transmission system control method in the present practiced formdescribed previously will be summarized as shown in Table of FIG. 7. Thepresent example is described as being a method of monitoring ofabnormality of the trunk line system in the decision procedure but thetrunk line system may be not only the transmission fiber but also asystem inclusive of the optical amplifier of the preceding stage.

In order to set the light outputs of the plurality of wavelengths λ1,λ2, λ3 and λ4 to the same level as far as possible, it is preferable touse the erbium added optical fiber 22 in which flatness of the gain isrealized as far as possible within the region of wavelengths to bemultiplexed.

In the present practical form, transmitting power of the probe lightbeam equals that of other wavelengths but for facilitation of detection,transmitting power of the probe light beam may be increased. Forexample, by making the output light signal of the transmitter of theprobe light beam +13 dBm and making the outputs of other signal lightbeams +10 dBm, the probe light beam can be about -20 dBm even when thesignal light beams decrease to about -23 dBm and therefore it can bedetected sufficiently by means of the light receiving unit 28. Bycontrolling the optical amplifying medium on the basis of the thusdetected value, the signal light beam outputs can be amplified from -23dBm to +10 dBm. Accordingly, the distance between an optical amplifierand the following optical amplifier can be increased to permitlonger-distance transmission.

In order to increase the mu-factor of the signal light beam gain,transmitting power of the probe light beam can otherwise be decreased.In this case, the output levels of the signal light beams are correlatedto the output level of the probe light beam and the respective signallight beam outputs are so controlled as to be set to +10 dBm by changingsetting of the reference voltages 31A and 31B for the probe light beam.

One of the wavelengths of the signal light beams can be used as theprobe light beam. By doing so, the optical transmission system of thepresent invention can be built easily without additionally providing anew probe light beam. In this case, it is preferable that the probelight beam be set to a long wavelength end or a short wavelength end ofsignal wavelengths to be multiplexed as described hereinbefore. When thenumber of wavelengths to be multiplexed is four, another probe lightbeam may be provided but as the number of wavelengths to be multiplexedincreases, the provision of a probe light beam other than signal lightbeams limits the amount of transmissive information and under thecircumstances, the transmissive information amount can be increased byusing one of the signal light beams commonly as a probe light beam. Inthe optical transmission system shown in FIG. 1, too, the number oftransmitters can be decreased, thus simplifying the circuitconstruction.

In the trunk line system in the practical system, a non-uniform decreaseoccurs in the multiplexed wavelengths, that is, in the light output ofthe probe light beam λp and the light outputs of the signal light beamsλ1, λ2, λ3 and λ4 owing to the non-linear optical effect calledfour-wave mixing. Accordingly, there occurs a difference between thelight input level of the probe light beam λp and that of each of thesignal light beams λ1, λ2, λ3 and λ4 and in spite of the fact that theprobe light beam λp is controlled to +10 dBm, there is a possibilitythat signal light beam outputs are so controlled as to deviate from +10dBm. But, by utilizing a non-dispersion shifted fiber for the trunk linesystem 9, the difference between the input level of the probe light beamand that of each of the signal light beams can be mitigated. Further, bytaking measures in which the wavelength distance is made to be narrowerbetween one of the signal light beam wavelengths, for example, λ1 andthe λp than between other signal light beams, the difference between theinput level of the probe light beam and that of each of the signal lightbeams can be mitigated.

According to the present practiced form, even when one signal light beamλ decreases or stops, other signal light beams are not affected therebyso as to be controlled to the constant level.

In the practical system, information pieces transmitted by the signallight beams λ1, λ2, λ3 and λ4 are often independent of each other.Accordingly, it is sufficiently thinkable that the signal light beamsdecrease or stop independently of each other. Further, in the practicalsystem to which the optical amplifier is applied, the number ofwavelengths to be multiplexed, for example, is not uniform and it issufficiently thinkable that, for example, the optical amplifier of themultiplexing system for the four signal light beams λ1, λ2, λ3 and λ4must be used for a multiplexing system for two signal light beams λ1 andλ2. Accordingly, it is important from the standpoint of system designthat transmitted signals are always controlled to a constant levelwithout depending on the signals which decrease or stop independently ofeach other. According to the present practiced form, even whenfour-wavelength multiplexing changes to two-wavelength multiplexing, thetransmitted signals can always be controlled to the constant level.

Further, even when one signal light beam, for example, λ1 light beamdecreases or extinguishes, the optical gain states (outputs) of thesignal light beams λ2, λ3 and λ4 can be held by the gain state (output)of the probe light beam λp by performing feedback control such that theoptical output of the probe light beam λp monitored by the lightreceiving unit 28 is kept to the predetermined value and hence they canbe maintained at the predetermined value and can be stabilized.Consequently, the light outputs of λ2, λ3 and λ4 wavelengths can becontrolled to the same level as that available before the λ1 signallight beam decreases or extinguishes. Accordingly, even when the numberof wavelengths to be multiplexed changes in the course of the use of theoptical amplifier, the signal light beams can be controlled stablywithout being changed in their control states. Similar control ispossible even when any of the wavelengths decrease or stop and aplurality of wavelengths decrease or stop. Even when the number ofmultiplexed wavelengths increases to λn, this holds true.

Accordingly, even with the number of wavelengths to be multiplexedincreased, such a situation can be handled without changing the systemand the circuit.

Further, from the viewpoint of reduction of laying cost, the signalmultiplexing number in the system can be increased or decreased inaccordance the necessity after laying, thus permitting a system capableof changing the wavelength multiplexing number with ease to be built.

Further, even when the probe light beam λp and the signal light beamsλ1, λ2, λ3 and λ4 change at a time, they can be controlled to theconstant value. In the practical system, the probe light beam λp andsignal light beams λ1, λ2, λ3 and λ4 often change at a time owing tofiber operation by the operator and deflection of the optical fiber.Even in such a situation, a variation in the probe light beam λp can besuppressed and can be controlled to the predetermined value and as aresult, other wavelengths, that is, the signal light beams λ1, λ2, λ3and λ4 together with the probe light beam can be maintained at thepredetermined value and stabilized.

Since the conventional scheme is one in which the total light output iscontrolled to a constant value, feedback control is performed such thatthe total light output of wavelengths λ2, λ3 and λ4 is kept to beconstant in the event that, for example, the λ1 signal light beamdecreases or extinguishes. Consequently, the light output of each of thewavelengths λ2, λ3 and λ4 is dispersed to increase by an amount ofdecrease or extinction of λ1 light output power. The amount multiplieseach of the λ2, λ3 and λ4 by about 1.3. When a plurality of wavelengthsextinguish, the light output further increases, making it impossible tokeep constant each output which is the essential control target.

In the event that the probe light beam per se becomes abnormal, all ofthe multiplexed light beams can be controlled to the constant value andtherefore, even in the event of such abnormality, recovery of the systemcan be deal with while maintaining a suitable system state.

Next, another practiced form of the optical amplifier will be describedwith reference to FIG. 3.

FIG. 3 is a block diagram showing the essential part of an opticalamplifier according to another practiced form of the present invention.Identical reference numerals to those in FIG. 2 designate identicalparts.

Automatic correcting means 43 includes a difference circuit 44, a memorycircuit 45 and a comparator 46. The difference circuit 44 is adapted toextract a difference between a light output of each of the wavelengthsλp, λ1, λ2, λ3 and λ4 which are monitored by the light receiving unit 27and a light output of λp monitored by the light receiving unit 28 and itcan remove a monitored value of the probe light beam λp from multiplexedlight beam monitored values to ensure output power of all of the signallight beams λ1, λ2, λ3 and λ4 to be monitored. All output power levelsof the signal light beams λ1, λ2, λ3 and λ4 upon initial adjustment arestored in the form of voltage values in a flash memory standing for thememory circuit 45. The comparator 46 compares the levels stored in thememory circuit 45 with all output power levels upon laying and thereference voltage 31A for the probe light beam is changed in accordancewith an error during the comparison.

With this construction, for example, on the assumption that when anoutput power level of all the signal light beams λ1, λ2, λ3 and λ4 uponinitial adjustment which is 4.0 V is stored, an output power level ofeach of all the signal light beams upon laying shifts to 3.5 V and avoltage value from the difference circuit 44 is 0.5 V, the referencevoltage 31A of the probe light beam is changed so that the above voltagebecomes 0 V. As a result, the output power of each of all the signallight beams can be set readily and steadily to the same level for theinitial adjustment period and the laying period.

According to the present practiced form, the output power of each of allthe signal light beams can be set to the same level for the initialadjustment period and the laying period.

Next, a third embodiment of the optical amplifier of the presentinvention will be described with reference to FIG. 4. In FIG. 4, too,parts described in connection with FIG. 2 are designated by identicalreference numerals. Essentially, FIG. 4 shows a modified example of thefirst wavelength separating means 24 in FIG. 2 and so parts other thanthe wavelength separating means have already been described inconnection with FIG. 2. Accordingly, a description of those parts willbe omitted.

In FIG. 4, an output signal from a port 3 of a 50:50 optical coupler 47is combined with an input light beam by means of a 5:95 optical coupler48. When abnormality occurs in the trunk line system 9 and the input tothe optical amplifier is stopped or decreased, the gain state of theerbium added optical fiber 22 is increased to maintain the monitor levelfor the probe light beam at a constant value and a spontaneous dischargelight beam standing for an optical noise component increases. Of thespontaneous discharge light beam branched by the 5:95 optical coupler23, only a light beam corresponding to the probe light beam λp passesthrough the optical filter 26 and combined with the input light beam bymeans of the 50:50 optical coupler 47 and 5:95 optical coupler 48, sothat a state is established in which a probe light beam is inputted asif normally. This ensures that a constant probe light beam istransmitted to the stage following the optical amplifier 8 or 10.

With the above construction, the alarm circuit 53 raises the alarm topermit abnormality in the trunk line system 9 preceding the opticalamplifier 8 or 10 to be indicated and at the same time, for thesucceeding trunk line system 9, the state is set up in which the probelight beam is inputted as if normally, so that in the absence ofabnormality especially occurring in the succeeding trunk line system,the alarm circuit of the succeeding optical amplifier 8 or 10 is notactuate and accordingly, the presence or absence of abnormality in thesucceeding trunk line system 9 can be detected independently ofdetection of abnormality in the preceding stage, thereby improving thereliability of the system.

When the 50:50 optical coupler 47 and 5:95 optical coupler 48 are notused, abnormality of the probe light beam occurring in the whole of thetrunk line system is detected by the individual optical amplifiers andall alarm circuits connected to the trunk line system raise the alarm,with the result that it takes time to investigate which stage of thetrunk line system the abnormality occurs in.

Since, inside the wavelength separating means 24, the wavelength band ofthe probe light beam substantially coincides with the pass-band of theoptical band-pass filter, it is preferable that the pass-band of theoptical band-pass filter be as narrow as possible.

The probe light beam to be transmitted to the succeeding stage mayindependently be provided in the optical amplifier by using, forexample, a semiconductor laser having the oscillation wavelength λp. Inthis case, the output from the semiconductor laser may be combined bythe 5:95 optical coupler 48.

According to the present practiced form, even when one signal light beamλ decreases or stops, this does not affect other signal light beams tomake it possible to control them to the constant level.

Further, the presence or absence of abnormality in the succeeding stageof the trunk line system can be detected independently of detection ofabnormality in the preceding stage without causing abnormality in thetrunk line to affect the succeeding stage and the reliability of thesystem can be improved.

Next, a fourth practiced form of the optical amplifier will be describedwith reference to FIG. 5.

FIG. 5 is a functional block diagram of an optical amplifier accordingto the fourth practiced form of the present invention.

The probe light beam λp and signal light beams λ1, λ2, λ3 and λ4 areinputted to an input terminal of the optical amplifier 8 or 10 throughthe same optical fiber. Most of the inputted probe light beam λp andsignal light beams λ1, λ2, λ3 and λ4 pass through the 5:95 opticalcoupler 18 standing for first light branching means so as to be inputtedto the optical amplifying medium 19.

The light beam guided to an input terminal of the erbium added opticalfiber 22 included in the optical amplifying medium 19 is amplifiedinside the erbium added optical fiber 22 and delivered from an outputterminal. Most of the delivered light beam passes through the 5:95optical coupler 23 standing for second light branching means and isdelivered from an output terminal of the optical amplifier 8 or 10 tothe trunk line system 9.

Part of the light beam is branched by the 5:95 optical coupler 23 so asto be inputted to wavelength separating means 54. The wavelengthseparating means 54 plays the role of the two of the wavelengthseparating means 24 and 34 in FIG. 2. The wavelength separating means 54includes a 50:50 optical coupler 49, an optical band-pass filter 50having a pass-wavelength band of λp±1 nm and a 50:50 optical coupler 51.

The light beam inputted from a port 1 of the 50:50 optical filter 51 is50:50 branched from ports 4 and 3 and the probe light beam λp and thesignal light beams λ1, λ2, λ3 and λ4 are delivered. The light beamdelivered out of the port 3 of the 50:50 optical coupler 51 is inputtedto the optical band-pass filter 50 through a port 1 and only the λplight beam standing for the probe light beam is passed through a port 2.

Of the thus separated light beams, the probe light beam λp and signallight beams λ1, λ2, λ3 and λ4 which are delivered out of the port 4 ofthe 50:50 optical coupler 51 are monitored by a first light receivingunit 27 and the λp probe light beam delivered out of the port 2 of theoptical band-pass filter 50 is inputted to a port 3 of the 50:50 opticalcoupler 49 and is 50:50 branched so as to be delivered out of ports 1and 2. A light beam delivered out of the port 2 is monitored by a secondlight receiving unit 28.

The controller 29 feedback-controls the drive current of the pump laser21 such that input monitor signals become constant. For detaileddescription of the circuit construction, reference can be made to FIG.2.

When the trunk line system is normal and the probe light beam isinputted normally, the drive current of the pump laser 21 isfeedback-controlled such that a light output of the probe light beam λpmonitored by the light receiving unit 28 is kept to be constant. Throughthis, a gain state inside the erbium added optical fiber 22 can be keptto be constant and hence optical gain states (outputs) of the pluralityof signal light beams λ1, λ2, λ3 and λ4 which are inputted concurrentlycan also be kept to be constant. As a result, light outputs of thesignal light beams λ1, λ2, λ3 and λ4 can be maintained at apredetermined value and can be stabilized. Here, the optical amplifier19 may be a semiconductor optical amplifier and in this case, the pumplaser 21 provides an exciting current.

On the other hand, the probe light beam λ and signal light beams λ1, λ2,λ3 and λ4 partly branched by the first 5:95 optical coupler 18 areinputted to the wavelength separating means 54. The 50:50 opticalcoupler 49 delivers the probe light beam λp and signal light beams λ1,λ2, λ3 and λ4 from the port 1 to a port 4. A light beam from the port 3of the 50:50 optical coupler 49 is inputted to the port 2 of the opticalbandpass filter 50 and only the λp light beam standing for the probelight beam is passed through the port 1. Of the thus separated lightbeams, the probe light beam λp and signal light beams λ1, λ2, λ3 and λ4which are delivered out of the port 4 of the 50:50 optical coupler 49are monitored by a third light receiving unit 37 and the probe lightbeam λp delivered out of the port 1 of the optical band-pass filter 50is monitored by a fourth light receiving unit 38.

The probe light beam λp is constantly generated. Therefore, when theprobe light beam is higher than the predetermined value, the connectionstate of the trunk line system 9 is determined to be good and the inputdetecting unit 33 delivers a command to the controller 29 so that theoutput monitored by the light receiving unit 28 may become constant andthe drive current of the pump laser 21 may be feedback-controlled suchthat the light output of the probe light beam λp is kept to be constant.

When the probe light beam λp monitored by the light receiving unit 38 islower than the predetermined value and the probe light beam λp andsignal light beams λ1, λ2, λ3 and λ4 which are monitored by the lightreceiving unit 37 are higher than the predetermined value, it isdetermined that the probe light beam λp by itself becomes abnormal andthe input detecting unit 33 transmits the second operation signal to thecontroller 29 so that the output of the light receiving unit 28 may becontrolled such that it becomes equal to the reference voltage. At thattime, the probe light beam λp by itself is determined to be abnormal inspite of the fact that the trunk line system 9 is normal and the drivecurrent of the pump laser 21 is feedback-controlled such that lightoutputs of the signal light beams λ1, λ2, λ3 and λ4 which are monitoredby the light receiving unit 37 can be kept to be constant. In thismanner, signal light beam output levels can be controlled such that theyare not raised abnormally in the absence of the probe light beam λp. Atthat time, given that the transmitted signal output of each of thesignal light beams λ1, λ2, λ3 and λ4 is 10 dBm, the output of each ofall the four multiplexed signal light beams λ1, λ2, λ3 and λ4 iscontrolled to +16 dBm.

In the event that the probe light beam by itself is suddenly stopped,the alarm is transmitted, at a speed of about 500 μs, from the operationdesignating circuit in the input detecting unit 33 to the selectors ofthe controller 29. Receiving this signal, the selectors are switched ata speed of about 50 μs. Since the erbium added optical fiber 22 has aresponse time of about 1 to 10 ms which is sufficiently slower than thealarm transmitting speed, the whole optical power can be controlledwithout causing the light output to be increased abruptly andexcessively.

By repairing the probe light beam while the second operation signal isgenerated, that is, while the light outputs of the signal light beamsλ1, λ2, λ3 and λ4 are kept to be constant, a suitable system state canbe maintained without making the signal transmission abnormal and thereliability of the system can be improved.

Instead of using the two reference voltages and switching them for use,a single reference voltage can alternatively be used by makingsensitivity of the light receiving unit 27 lower than that of the lightreceiving unit 28 such that, for example, an output electric signal hasthe same value when a light signal of 10 dBm is inputted to the lightreceiving unit 28 as when a light signal of 16 dBm is inputted to thelight receiving unit 27.

When the probe light beam λp monitored by the light receiving unit 38 islower than the predetermined value and the probe light beam λp andsignal light beams λ1, λ2, λ3 and λ4 which are monitored by the lightreceiving unit 37 are also lower than the predetermined value, it isdetermined that the trunk line system is abnormal and the operationdesignating circuit in the input detecting unit 33 sends the thirdoperation signal to the alarm circuit 53 to cause it to raise the alarm.

Alternatively, control may be performed such that the third operationsignal is used to lower the drive current of the pump laser 21 so as toprevent superfluous optical amplification.

In order to set the light outputs of the plurality of wavelengths λ1,λ2, λ3 and λ4 to the same level as far as possible, it is preferable touse the erbium added optical fiber 22 in which flatness of the gain isrealized as far as possible within the region of wavelengths to bemultiplexed.

In the present practiced form, transmitting power of the probe lightbeam equals that of other wavelengths but for facilitation of detection,transmitting power of the probe light beam may be increased. Forexample, by making the output light signal of the transmitter of theprobe light beam +13 dBm and making the outputs of other signal lightbeams +10 dBm, the probe light beam can be about -20 dBm even when thesignal light beams decrease to about -30 dBm and therefore it can bedetected sufficiently by means of the light receiving unit 28. Bycontrolling the optical amplifying medium on the basis of the thusdetected value, the signal light beam outputs can be amplified from -30dBm to +10 dBm.

In order to increase the mu-factor of the signal light beam gain,transmitting power of the probe light beam can otherwise be decreased.In this case, the output levels of the signal light beams are correlatedto the output level of the probe light beam and the respective signallight beam outputs are controlled such that they are set to +10 dBm bychanging setting of the reference voltages 31A and 31B for the probelight beam.

One of the wavelengths of the signal light beams can be used as theprobe light beam. By doing so, the optical transmission system of thepresent invention can be built easily without additionally providing anew probe light beam. In this case, it is preferable that the probelight beam be set to a long wavelength end or a short wavelength end ofsignal wavelengths to be multiplexed as described previously. When thenumber of wavelengths to be multiplexed is four, another probe lightbeam may be provided but as the number of wavelengths to be multiplexedincreases, the provision of a probe light beam other than signal lightbeams limits the amount of transmissive information and under thecircumstances, the transmissive information amount can be increased byusing one of the signal light beams commonly as a probe light beam. Inthe optical transmission system shown in FIG. 1, too, the number oftransmitters can be decreased, thus simplifying the circuitconstruction.

In the trunk line system in the practical system, a non-uniform decreaseoccurs in the multiplexed wavelengths, that is, in light outputs of theλp and the λ1, λ2, λ3 and λ4 owing to the non-linear optical effectcalled four-wave mixing. Accordingly, there occurs a difference betweenthe light output level of the λp and that of each of the λ1, λ2, λ3 andλ4 and in spite of the fact that the probe light beam is controlled to+10 dBm, there is a possibility that signal beam outputs are controlledsuch that they deviate from +10 dBm. But, by using a non-dispersionshifted fiber for the trunk line system 9, the difference between theinput levels, fed to the optical amplifier, of the probe light beam andsignal light beams can be mitigated. Further, by taking measures inwhich the wavelength distance is made to be narrower between one of thesignal light beam wavelengths, for example, λ1 and the λp than betweenother signal light beams, the difference between the input levels, fedto the optical amplifier, of the probe light beam and signal light beamscan be mitigated.

According to the present practiced form, even when one signal light beamλ decreases or stops, other signal light beams are not affected therebyso as to be controlled to the constant level.

With the above construction, the number of optical filters and opticalcouplers can be reduced and the construction of the wavelengthseparating means can be simplified.

Next, another practiced form of the optical transmission system of thepresent invention will be described with reference to FIG. 6.

The system construction of FIG. 6 differs from FIG. 1 in that atransmission system uses a plurality of probe light beams.

A probe light beam having a wavelength λp is used in correspondence tosignal wavelengths λ1 to λ4 and the mu-factor of an optical amplifier 8is controlled such that each of the signal light beams λ1 to λ4 and theprobe light beam λp are set to +10 dBm or more. At the same time, aprobe light beam having a wavelength λp' is used in correspondence towavelengths of signal wavelengths λ1' to λ4' and the mu-factor of theoptical amplifier 8 is controlled such that each of the signal lightbeams λ1' to λ4' and the probe light beam λp' are set to +10 dBm ormore. Consequently, the separate probe light beams λp and λp' are usedto control the mu-factor of the optical amplifier 8 such that each ofthe signal wavelengths λ1 to λ4 and each of the signal wavelengths λ1'to λ4' are steadily set to +10 dBm or more.

While the number of wavelengths in the figure is four in each group, thewavelength number is not always limited to four but may be different forthe respective groups. The number of the separately provided probe lightbeams is two in the figure but it is not limitative.

If in the optical transmission system a single probe light beam is usedto control all of other signal light beams when the number ofwavelengths of signal light beams increased, then there will be apossibility that a difference occurs between power of the probe lightbeam and that of other signal light beam wavelengths and a wavelengthoccurs which is unstable to the probe light beam. Especially when thewavelength of the probe light beam is distant from wavelengths of signallight beams, such a phenomenon as above is liable to occur. With theconstruction of FIG. 6, however, an increased number of wavelengths aredivided into groups each of which is separately monitored by the probelight beam and therefore, all signals can steadily be amplified anddelicate and fine feedback control can be ensured.

Through this, an optical transmission system of high reliability andsafety can be built.

According to the present invention, in the optical amplifier and theoptical transmission system using it, the individual wavelength outputscan be controlled without affecting amplification of other light signalsand without greatly affecting the signal transmission even when thenumber of signal wavelengths to be multiplexed changes.

I claim:
 1. An optical amplifier comprising:an optical amplifying mediumto which input multiplexed light beams are guided; an exciting sourcefor exciting said optical amplifying medium; light branching means forbranching part of the multiplexed light beams amplified by said opticalamplifying medium; light separating means for separating a probe lightbeam from multiplexed light beams branched by said light branchingmeans; light receiving means for detecting an output of the probe lightbeam separated by said light separating means and converting thedetected output into an electric signal; control means for controllingan amount of operation of said exciting source on said optical mediumsuch that an output of said light receiving means becomes constant;second light receiving means for detecting outputs of multiplexed lightbeams branched by said light branching means; second branching meansarranged to precede said optical amplifying medium and operative tobranch part of the multiplexed light beams guided to said opticalamplifying medium; second light separating means for separating theprobe light beam from the multiplexed light beams branched by saidsecond branching means; third and fourth light receiving means fordetecting the output of the probe light beam branched by said secondseparating means and the outputs of the multiplexed light beams branchedby said second separating means, respectively; and input detecting meansfor delivering a command to said control means when the output of theprobe light beam detected by said third light receiving means is lowerthan a predetermined value and the outputs of the multiplexed lightbeams detected by said fourth light receiving means are higher than apredetermined value, said control means being operative to control theamount of operation of said exciting source on said optical medium suchthat the outputs of said second light receiving unit become constant. 2.An optical amplifier according to claim 1 further comprising alarm meansfor informing abnormality, wherein said input detecting means delivers asignal to said alarm means when the output of the probe light beam andthe outputs of the multiplexed light beams are lower than thepredetermined values, respectively, so as to actuate said alarm means.3. An optical amplifier according to claim 1 further comprisingcorrecting means for calculating a difference between outputs of saidfirst and second light receiving units and correcting a reference value,included in said control means and used to control the probe light beamto a constant value, in such a manner that the difference becomes apredetermined value.
 4. An optical amplifier according to claim 3,whereinsaid light separating means includes first and second opticalcouplers each having four ports and an optical filter for selectivelypassing a light beam of a predetermined wavelength; the multiplexedlight beams branched by said light branching means are guided to thefirst port of said first optical coupler and multiplexed light beamsdelivered out of the fourth port are detected by said second lightreceiving means; the multiplexed light beams branched by said lightbranching means are guided to the first port of said first opticalcoupler, multiplexed light beams delivered out of the third port areguided to said optical filter and further guided to the third port ofsaid second optical coupler and the probe light beam delivered out ofthe second port is detected by said second light receiving unit;multiplexed light beams branched by said second light branching meansare guided to the first port of said second optical coupler andmultiplexed light beams delivered out of the fourth port are detected bysaid fourth light receiving unit; and multiplexed light beams branchedby said second light branching means are guided to the first port ofsaid second optical coupler, multiplexed light beams delivered out ofthe third port are guided to said optical filter and further guided tothe third port of said first optical coupler, and the probe light beamdelivered out of the second port is detected by said first lightreceiving unit.